1. Field of the Invention
This invention relates in general to devices for clamping parts to be machined, and in particular to a hydraulically actuated internal clamp for clamping billets for cutting operation on numerical control machines.
2. Description of the Prior Art
In the aircraft industry, many parts are fabricated from large slabs of aluminum known as billets. An initial step is cutting the billets with multiple head, numerical controlled, milling machines. The billet is clamped to a fixture below the milling machine head by various means. The clamps usually comprise bolts, straps or other types of mechanical clamps.
These types of clamps have a disadvantage in that the machine cutter has to go around the clamp, which is especially a problem in numerical control machining using multiple hand milling machines. Also, the clamps may have to be moved, removed or added during the machining sequence. Often, the machine operator must physically climb up on the machine bench to do anything with the clamps. This creates a loss in time and a hazardous safety problem. Many clamps and bolts used on machine tools are different sizes, adding to the confusion of selecting the correct device. This causes shutdowns to find lost clamps or replace missing components. Any loose or removable part on a machine tool is a serious obstacle to production requirements. Exposed surface clamping incurs a risk of a high speed cutter striking the clamp due to machine malfunction or error, also.
Locking collets have been known for some time. Generally, a locking collet comprises a tubular mandrel through which an expander stem is carried. Moving the expander stem in one direction expands the mandrel to lock the part to the mandrel. Normally, the expander stem is part of a spindle of the machine. Locking collets of this type have been used for machining gears and other parts.
While a locking collet may be acceptable for its particular task, in heavy cutting operations, such as concerned herein, a conventional locking collet would not have sufficient holddown power. Also, most locking collets protrude past the workpiece within which they are secured, creating an obstacle if attempted to be used with a numerical control milling machine. Moreover, these locking collets do not rigidly secure a workpiece to a flat supporting surface for cutting operations.